Why are 3 phase motors with VFD so popular in the UK?

[larry phelan 1Participant @larryphelan1]

Like Hopper, I seldom feel the need to be changing the speed on my drill, and anyway it takes only a moment to change, so no big deal. I like to keep things simple and uncomplicated [less to go wrong ] and if I dont see any real advantage, I dont change. I agree 3 ph motors are smoother running, but I could live with single phase if I had to. Some of my machines are single phase anyway, it,s just that before my last move, I had 3ph for some bigger woodworking machines and when I moved to the sticks, I needed to provide for them, hence a converter.

I get the feeling that for many people, it,s just something they feel they "need" and "must have" in order to fit in.

No doubt I will be told off in no uncertain manner, but then, I,m used to that !

[HopperParticipant @hopper]

Posted by Bazyle on 21/01/2023 09:51:04:

Seems like there is a business opportunity for an enterprising fellow in Oz.

Could be. A very small market the Australian hobby market though. Still, surprising none of our major industrial/home workshop suppliers like Hare and Forbes have not started importing them. I have noticed that things like 3/4HP single phase motors are more expensive here than the same in 3 phase, probably because industry uses 3 phase so more are imported. So it would only be a matter of organising the controller unit side of it. If they were readily available as a plug and play unit, I am sure quite a few guys fixing up old Myfords and Hercuses (Aussie Boxfordy SouthBend clone) would do the upgrade to VFD.

[John Doe 2Participant @johndoe2]

This is my understanding:

A single phase motor produces zero torque twice every AC cycle of the mains waveform, when that crosses zero volts. So there will be a deep torque ripple of 2 x mains frequency.

Compare that to a 3 phase motor; As each phase AC waveform goes "over top dead centre" and torque from that winding starts falling, the next phase – 120° behind it – is approaching "top dead centre", so its torque is increasing, and so-on; So a 3 phase motor will produce torque throughout each rotation and have only a very shallow torque ripple, which will be 3 x the frequency of a single phase motor.

So a 3 phase motor will have much higher torque, delivered much more smoothly.

You can think of this as a being similar to single cylinder 'thumper' internal combustion engine compared to a 3 cylinder engine: the more cylinders, the smoother the torque delivery.

Not being a machinist, I don't know if the torque ripple from a single phase motor is likely to cause more chatter etc. at the cutting bit, compared to a 3 phase motor ?

 

As an aside, I wonder if the addition of a flywheel onto a single phase motor would help? It would smooth out the torque ripple a bit and provide a more gentle slow start, though not a quick stop of course.

 

Edited By John Doe 2 on 21/01/2023 13:09:03

[Harry WilkesParticipant @harrywilkes58467]

My answer would be due to have arthritis in both wrist's the advantage of a VDF is 'reverse'

H

[Martin ConnellyParticipant @martinconnelly55370]

The effect of a flywheel large enough to reduce the acceleration of a single phase motor may be a large current draw until it is up to speed. This is not great for the wiring or the motor unless it has all been designed for such a system. Then when power is cut off the system will act like a generator until the motor is at rest so you may need a system designed to dump that energy into a resistor (a DC brake in effect). Turning a large lump of metal may cause the same effect, they can take a few turns to stop if you have a lathe with no brake and no clutch. That is why larger lathes tend to have both a brake and a clutch.

Martin C

[John HaineParticipant @johnhaine32865]

Posted by John Doe 2 on 21/01/2023 13:00:18:

This is my understanding:

A single phase motor produces zero torque twice every AC cycle of the mains waveform, when that crosses zero volts. So there will be a deep torque ripple of 2 x mains frequency.

…..

Edited By John Doe 2 on 21/01/2023 13:09:03

Um. No. A single phase stator generates a static but alternating magnetic field that can be thought of as the sum of two contra-rotating fields of half the strength. Both of these induce currents in the rotor and in turn torque but in opposite directions. Once the rotor is turning the field rotating in the same direction is seen by the rotor as having a lower frequency and since the rotor current at 50Hz is limited by inductance, more current flows at the difference frequency. Likewise the rotor sees the contra-rotating field as having a higher frequency so less current flows. Overall there is a constant torque (once the rotor has reached nearly synchronous speed) on which is superimposed a torque ripple at the sum of the actual speed and the drive (50Hz say). The ripple is much smaller than the torque because the sum frequency is nearly 100Hz and the inductive impedance is much higher, so the current is much less.

[old martParticipant @oldmart]

Here is a basic example for a 1hp motor and a VFD to suit. This does not include the switches for remote control or wiring, so £35 should be added. Motor and VFD £225 which includes VAT but not delivery. This firm has a "quick start guide" for some VFD's which makes wiring, controls and programming very easy.

**LINK**

[duncan webster 1Participant @duncanwebster1]

If you Google Newton Tesla they do prewired vfd motor control panel packages tailored for many popular lathes. I've bought 3 packages from them, not the cheapest but very helpful.

[Anonymous]

I went 3ph with VFD on my ML7. Much quieter than the original single phase motor. I can also run up to 2x motor speed but rarely do. Much handier is the ability to run at very slow speeds (down to zero) by just turning a knob – without fiddling with the belts – for such things as threading with tailstock die. (There's always been enough torque to do that for the small threads that I'm generally interested in …. I can always change belts as well if necessary for larger threads.)

Wouldn't be without it.

[Simon CollierParticipant @simoncollier74340]

I’ve lived all these years in this world and never once noticed a torque ripple. I’ll have to start paying attention.

[HopperParticipant @hopper]

Posted by Simon Collier on 21/01/2023 22:02:46:

I’ve lived all these years in this world and never once noticed a torque ripple. I’ll have to start paying attention.

It's what makes the spanners rattle when you leave them in the drip tray of the lathe. My solution is to hang the spanners back up on the rack!

[SillyOldDufferModerator @sillyoldduffer]

Posted by Simon Collier on 21/01/2023 22:02:46:

I’ve lived all these years in this world and never once noticed a torque ripple. I’ll have to start paying attention.

Not noticing isn't the same as not existing! But although single-phase motors are imperfect. they're not obvious rubbish either. The ripple tends to be minimised by spring in the drive belt and flywheel effect, and it's small unless the motor is pushed hard by taking heavy deep cuts. Do that with a difficult metal and take a close look at the surface with a loupe.

If I owned a single-phase Myford in good order I wouldn't bother to change the motor. However, if it needed a new one, I'd go 3-phase for less vibration, better performance and speed control. 3-phase motors deliver more torque too. I don't think anyone who has successfully upgraded to a 3-phase motor has ever regretted it.

3-phase and VFDs are pushing out single-phase motors for many commercial purposes. The motors are cheaper, more reliable, more efficient, and then VFDs save money with controlled soft starts. Big savings whenever a large number of motors stop and start repeated;ly, because the heavy current surge when a single-phase motor starts is a waste of money. A hobby lathe owner is unlikely to notice the waste, but a factory or building manager would.

Dave

[John Doe 2Participant @johndoe2]

Thanks John (Haine); I got most of that except for the constant torque bit at the end. Surely with only a single AC phase applied to the stator, (no capacitors etc) at the instant in time when the AC waveform passes through zero volts, the magnetic field will also – momentarily – fall to zero ? Or is it maintained by induction ?

Obviously the rotor keeps going owing to its own inertia, so it does not actually stop.

[Clive IndiaParticipant @cliveindia]

Posted by PatJ on 20/01/2023 23:12:57:
The reason I don't use these is as follows:

Snip..
3. I am old-school, and I prefer to use as few electronics as possible in the shop.
….Snip

Considering all your misgivings it's better you don't have one and that's fine but….

Posted by Peter Greene 🇨🇦 on 21/01/2023 19:57:32:

I went 3ph with VFD on my ML7. Much quieter than the original single phase motor. I can also run up to 2x motor speed but rarely do. Much handier is the ability to run at very slow speeds (down to zero) by just turning a knob – without fiddling with the belts – for such things as threading with tailstock die. (There's always been enough torque to do that for the small threads that I'm generally interested in …. I can always change belts as well if necessary for larger threads.) Wouldn't be without it.

Yes – I did that with an S7 and it's great – would be weary of exceeding speed though.

Posted by SillyOldDuffer on 21/01/2023 22:57:11:
Snip…
If I owned a single-phase Myford in good order I wouldn't bother to change the motor. However, if it needed a new one, I'd go 3-phase for less vibration, better performance and speed control. 3-phase motors deliver more torque too. I don't think anyone who has successfully upgraded to a 3-phase motor has ever regretted it.
Snip…Dave.

Very much agree.

[Howard LewisParticipant @howardlewis46836]

Single phase AC motors are resiliently mounted, to reduce vibration.

It has been noted that under certain circumstances, a single phase powered lathe will produce a faint helix on the work. (Presumably caused by ma regular variation in speed )

As a simple mechanical engineer I think of a single phase motor (On 50 Hz mains ) producing 50 impulses per second. A three phase motor fed by the same 50 Hz supply will deliver 450 impulses per second, which suggests a much smoother delivery of power.

Industry has used 3 phase motors for many years, because of the smoother delivery of power

Presumably it could be argued that 5 hp 3 phase motor will draw the same average current as a 15 hp single phase machine. to save on capital and power costs.

A VFD usually delivers 3 phase power (often at 240 volts ) from a single phase 240 volt input.

Mine is fed via a suppressed socket, since I do not wish to cause problems within my household, or any other fed from the same phase, by any spikes being fed back into the mains. After nearly 20 years of use reported, no problems.

The motor concerned is 1 1/2 hp (Replacing the original 2 hp single phase ), and is Australian made!

Howard

[Martin ConnellyParticipant @martinconnelly55370]

Howard, power is volts times amps. For a 15hp 3 phase motor the phase currents will be 1/3 the current of a single phase 15HP motor at the same voltage. This (simplified) means that at full power a 1.2kW single phase motor at 240V motor will require a current of 0.5A in its single phase wiring. A 1.2kW 3 phase motor fed 240V (line to line as in a VFD) will require 3 currents of 0.5A/3 to give the same 1.2kW at full power. The current to the VFD will be slightly higher than the single phase current due to losses in the VFD.

Martin C

[Michael Cross 4Participant @michaelcross4]

I can't be the only person who has noticed that machines fitted with single phase motors attract a significant premium over three phase machines so obviously not everyone sees or understands the benefits. It's usually cheaper to fit a used three phase machine with an inverter than to source a single phase machine. It's also usually much easier to equip a three phase machine with a VFD than to replace the motor with a single phase one because single phase motors are much bigger for the same power, so if you go that way you're quickly into new mounts, adjusting pulleys and so on.

The benefits of changing speed using a VFD are sometimes exaggerated as if you do this you don't get the increase in torque that you would from mechanical gearing. This is why Variable Speed DC motors (which are actually three phase AC motors) stall so easily when you run them slowly. Any three phase motor will do the same if you turn the frequency down and then push it. Running significantly outside of the designed frequency is also not great for your motor, particularly if this leads to stalling, which robs you of more torque, stalls more easily and the death spiral continues. It's fine as a way of fine tuning your speeds but it's not the ideal way to adjust your speeds across a wide range. That said it is convenient and if you're well within your machine's capability then you might reasonably accept the compromise.

Regional differences in the use of VFDs are more likely down to availability of used machines from industry than this or that place being 'backward'. Here in the UK we are blessed with a plentiful supply of both good used industrial machines and VFDs so putting the two together makes perfect sense. I don't know if that's the case in OZ but that sort of reason will surely explain any disparities.

[duncan webster 1Participant @duncanwebster1]

Posted by Howard Lewis on 22/01/2023 15:45:19:

……

Presumably it could be argued that 5 hp 3 phase motor will draw the same average current as a 15 hp single phase machine. to save on capital and power costs

…….

Howard

3 phase power is 1.732*V * I, but you have to pay for all 3 phases.

[SillyOldDufferModerator @sillyoldduffer]

Always good to look at stuff from the other way. What do single-phase induction motors bring to the party?

Single-phase is conveniently simple for domestic purposes like heating, lighting and electronics, but getting it to turn an electric motor is difficult. To spin, the motor requires the designer to create two magnet fields that oppose and attract in concert as the rotor turns. And it has to do this in a way that allows the motor to start under a reasonable load as well as to stay spinning once it's going.

So the only advantage of single-motors is they run off single-phase domestic AC power and do so reasonably well. That was vital back in the day, but the motor is a clever compromise rather than the best of all possible motors. Look closely and they're a bit poo compared with other types:

• Complicated. Start and Run Windings, Centrifugal Switch and one or two stressed capacitors.
• Unreliable because they're complicated
• Inefficient because the start winding may not contribute to power output.
• Prone to overheating due to inefficiency
• Bulky
• Low torque (turning power)
• Heavy current draw on start
• Hard to speed control
• High vibration

In comparison a 3-phase motor is remarkably simple, smooth, efficient and reliable – much less to go wrong. Starting torque is good and they self-start without any special arrangements. They're cheaper to make and take up less space. These advantages counted for nothing when homes only had single-phase power available. Not so today, when 3-phase can be produced from single phase. When done electronically, it's easy to control speed by varying frequency, plus the electronics come with host of other opportunities for getting the best out of motors by managing the power feed.

For many applications, including machine tools, 'Brushless DC' motors are increasingly challenging 3-phase, even though 3-phase motors are high performers. Brushless DC has been around for years but during this century the availability of affordable super-magnets and power handling electronics has made them much more popular.

Single-phase motors used to the obvious first choice. Not so now. I'd say they're 4th choice on a new machine tool.

Dave

[John HaineParticipant @johnhaine32865]

"…if you do this you don't get the increase in torque that you would from mechanical gearing. This is why Variable Speed DC motors (which are actually three phase AC motors) stall so easily when you run them slowly. Any three phase motor will do the same if you turn the frequency down and then push it. "

See this website: http://thinkvert.com/variable-frequency-drive-affects-on-torque-horsepower/

**LINK**

As the frequency is reduced below design speed the VFD can drive the motor to generate constant torque down to low speeds. Imagine a motor which is clamped to the rotor can't turn and the stator is driven with current at the slip frequency so that it produced the same field in the air gap as if the motor is running at its maximum torque point. The stator will see exactly the same magnetic conditions and must generate the same torque. The speed of course decreases and therefore the power, but for many jobs torque is what's wanted at low speed – for example tapping.

Most of the cheap machine tools use permanent magnet DC motors and thyristor controls (or sometimes MOSFETs). These behave differently. Some DC drives use brushless motors, which I believe generally use permanent magnet rotors. The field windings are 3 phase to generate rotating magnetic fields to drag the rotor round. The torque depends on the stator field and the physical angle between this and the rotor magnets and is independent of speed.

**LINK**

[Tony JeffreeParticipant @tonyjeffree56510]

As John's link above shows, a VFD/3-phase motor setup will generate broadly constant torque below its "normal" operating frequency (60Hz in the example shown) and broadly constant power above that frequency. If you are wanting to maximise the amount of metal that you can remove, then you need to maximise power, so you want to be operating in the middle/right hand half of that curve, but for some other operations (as John observes) like tapping where torque is needed but relatively little power, then operating in the left half is just fine, and particularly for operations such as tapping, the speed control is very helpful indeed. I often tap at low speed under power in my ML-7 – haven't yet broken a tap that way.

Having had one of the first Newton Tesla kits fitted to my lathe for a good many years now, what I find in practice is that the variable speed allows me to use the middle speed range of the lathe for most of the work that I do, and that it is relatively rare that I need to shift pulleys up or doen (or engage back gear, for that matter).

Fitting a VFD to an existing 3-phase setup is significantly cheaper than other options for providing a 3-phase supply from single phase, so even if you plan not to use the variable speed option (always run at 50 Hz and change speeds via the pulleys) then it is still a no brainer if your mains supply is single phase only.

[Michael Cross 4Participant @michaelcross4]

My point is that:

you don't get the increase in torque that you would from mechanical gearing

Constant torque is one thing, but mechanical gearing increases your toque and adjusting the frequency doesn't, so for a given speed you end up with less torque if you use your VFD to reduce speeds than if you shift the pulleys / gears.

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Michael Cross 4 on 23/01/2023 17:08:32:

My point is that:

you don't get the increase in torque that you would from mechanical gearing

Constant torque is one thing, but mechanical gearing increases your toque and adjusting the frequency doesn't, so for a given speed you end up with less torque if you use your VFD to reduce speeds than if you shift the pulleys / gears.

…all of which is consistent with what I wrote.

My view is that with a VFD you really do get the best of both worlds. Use the VFD to change speeds where that is convenient/useful/appropriate; where it isn't, you still have the option of running the motor at "normal" speed (where it operates at max power and max torque) and increasing the available torque by using the pulleys/gears.

[old martParticipant @oldmart]

A lot of people who want the convenience of variable speed which a VFD gives are simply too lazy to bother changing gears or belts. My view is that the VFD complements the mechanical advantage provided by manual gearing.

[Michael Cross 4Participant @michaelcross4]

Yes Tony I completely agree with you I think our views on this are very close together. I was addressing John.

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